Dual layer etch stop barrier

ABSTRACT

59 A method for reactive ion etching of SiO 2  and an etch stop barrier for use in such an etching is provided. A silicon nitride (Si x N y ) barrier having a Si x  to N y  ratio (x:y) of less than about 0.8 and preferably the stoichiometric amount of 0.75 provides excellent resilience to positive mobile ion contamination, but poor etch selectivity. However, a silicon nitride barrier having a ratio of Si x  to N x  (x:y) of 1.0 or greater has excellent etch selectivity with respect to SiO 2  but a poor barrier to positive mobile ion contamination. A barrier of silicon nitride is formed on a doped silicon substrate which barrier has two sections. One section has a greater etch selectivity with respect to silicon dioxide than the second section and the second section has a greater resistance to transmission of positive mobile ions than the first section. One section adjacent the silicon substrate has a silicon to nitrogen ratio of less than about 0.8. The second section, formed on top of the first section is formed with the ratio of the silicon to nitrogen of greater than about 0.8. Preferably the two sections together are from about 50 to about 100 nanometers thick.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to the manufacture of integratedcircuit (I/C) chips and particularly to the fabrication or processing ofa silicon substrate to form the circuitry for the I/C chip. During onestage of manufacture of I/C chips, a silicon dioxide layer is appliedover a silicon substrate. The silicon dioxide must be etched at variousplaces to provide openings to the substrate for electrical connections.One conventional technique of etching is by means of reactive ionetching (RIE). With reactive ion etching it is conventional to providean etch stop barrier between the silicon substrate and the silicondioxide layer formed thereon. One conventional etch stop barrier issilicon nitride (Si_(x)N_(y)). These silicon nitride barriers areconventionally deposited by low pressure chemical vapor deposition(LPCVD) utilizing conventional equipment. In one embodiment mixtures ofsilane (SiH₄) and ammonia (NH₃) are utilized as an ambient to providethe necessary silicon and nitrogen moieties for the formation of thesilicon nitride.

[0002] However, it has been found in the past that there were variationsfrom process to process of forming the Si_(x)N_(y) barrier in theeffectiveness of the nitride barrier in its selectivity with respect toSiO₂ when reactive ion etching the SiO₂. When etching SiO₂ it isdesirable to have as much selectivity as possible of the etch stop withrespect to the SiO₂ so as to allow a minimum thickness of the etch stopto be applied. It was also found that there were variations in theresulting barrier in the effectiveness of the silicon nitride to preventpassing of positive mobile ions (PMI) which may occur during subsequentprocessing due primarily to contaminants introduced into the SiO₂ layer.Positive mobile ion contamination (PMIC) such as in a gate oxide of CMOSdevices must be reduced to a minimum. Thus a requirement of the siliconnitride barrier is that it act to effectively block positive mobile ionsfrom penetrating into the substrate during subsequent processing steps.

[0003] Therefore it is desirable to provide a silicon nitride barrierthat is both highly selective to etching of SiO₂ and also effective toblock the passage of positive mobile ions in subsequent processingsteps.

SUMMARY OF THE INVENTION

[0004] According to the present invention, a method for reactive ionetching of SiO₂ with an etch stop barrier for use in such an etching isprovided. It has been found that a silicon nitride (Si_(x)N_(y)) barrierhaving a Si_(x) to N_(y) ratio (x:y) of less than about 0.8 andpreferably the stoichiometric amount of 0.75 provides excellentresilience to positive mobile ion contamination, but poor etchselectivity. However, a silicon nitride barrier having a ratio of Si_(x)to N_(y) (x:y) of 1.0 or greater has excellent etch selectivity withrespect to SiO₂ but a poor barrier to positive mobile ion contamination.The technique of the present invention includes providing a substratewhich conventionally is a doped silicon substrate, and forming a barrierof silicon nitride on the substrate which barrier has two sections orlayers. One section has a greater etch selectivity with respect tosilicon dioxide than the second section and the second section has agreater resistance to transmission of positive mobile ions than thefirst section. Preferably the two sections are formed by forming onesection, referred to as the lower section adjacent to silicon substratewith a silicon to nitrogen ratio of less than about 0.8 and preferablyabout 0.75 which is the stoichiometric ratio of silicon to nitrogen. Thesecond section, or upper section is preferably formed with the ratio ofthe silicon to nitrogen of greater than about 0.8 and preferably atleast about 1.0. Preferably the two sections together are from about 50to about 100 nanometers thick and in the preferred embodiment, eachsection is about 25 to 50 nanometers thick.

DESCRIPTION OF THE DRAWING

[0005]FIG. 1 is a graph of the etch rate of silicon nitride(Si_(x)N_(y)) in Ar:CHF₃CF₄ at various silicon to nitrogen ratios (x:y)of the silicon nitride;

[0006]FIG. 2 is a bar graph showing the V_(t) shift of a substrate afterreactive ion etching using silicon nitride (Si_(x)N_(y)) barriers ofvarious ratios of silicon to nitrogen (x:y);

[0007]FIG. 3 is a graph similar to FIG. 2 graphing the positive iondensity in the substrate as a function of the ratio of the silicon tonitrogen (x:y) in silicon nitride; and

[0008]FIGS. 4A through 4G show the steps of the method of the presentinvention somewhat diagrammatically.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009] The use of silicon nitride as an etch stop barrier is well knownin the art especially for stopping the etch during reactive ion etching(RIE) of silicon dioxide disposed over a silicon or doped siliconsubstrate in the manufacture of integrated circuit chips. Reactive ionetching is used in chip manufacturing to form openings through thesilicon dioxide so as to provide access to the substrate. Typically theopening will be filled with metal such as tungsten or other metal as iswell known. In etching the silicon dioxide an etch stop layer is used soas to allow the etching to stop or essentially terminate once theetching has penetrated through the silicon dioxide layer. Expressedanother way, when the etching has pierced the silicon dioxide layer itis desired that the etching not continue to any significant extent. Thebarrier layer of etch stop material is to ensure that the etch stopssubstantially uniformly at all the various locations being etchedthrough the silicon dioxide. Thus, one of the principal requirements ofthe etch stop material is that it have a relatively high selectivity ofetching with respect to the material which is intended to be etched i.e.silicon dioxide. Expressed another way, once the silicon dioxide hasbeen etched it is desirable that there be very little etching takingplace after that.

[0010]FIG. 1 shows the etching rate of Si_(x)N_(y) in nanometers perminute using an AME 5000 tool with Ar:CHF₃ atmosphere at various ratiosof silicon to nitrogen in a silicon nitride (Si_(x)N_(y)) barrier. Ascan be seen, when the ratio of silicon to nitrogen is 0.75 (which is thestoichiometric ratio) the etch rate is between 140 and 160 nanometersper minute, but as the ratio of silicon to nitrogen increases, this etchrate decreases dramatically to a point where when the ratio of Si to Nis about 1.0 the etch rate has dropped down to about 20 nanometers perminute. With a ratio greater than 1.0 no improvement in the etch rateresistance is achieved. Thus, based on this particular characterization,in order to get the lowest etch rate of silicon nitride and thus thehighest etch selectivity, it is desirable to have a ratio of silicon tonitrogen of at least about 1.0.

[0011] However, in subsequent processing during chip manufacture therecan be generated positive mobile ions (PMI), in particular Na⁺ and K⁺,principally from contamination in the SiO₂ layer. If these positive ionsdiffuse even in small amounts into the silicon substrate they can causesignificant degradation of the substrate material in some structures.Thus, it is desirable and often even necessary that these ions beessentially excluded from penetrating the barrier and diffusing into thesubstrate. FIGS. 2 and 3 show the amount of diffusion of positive mobileions especially sodium (Na⁺) as measured by Vt Shift (mV) shown in FIG.2 and ion density in 10¹⁰ Ions/cm² shown in FIG. 3 in substrates withSi_(x)N_(y) nitride barriers having various ratios of Si to N in thesilicon nitride. At a Si to N ratio of 1.05 there is a very high numberof mobile ions passing through the silicon nitride barrier, and even ata ratio of 1.0 there is an appreciable amount of these ions penetrating;indeed even at a ratio of silicon to nitrogen of 0.8 there is asignificant amount of PMIC (positive mobile ion contamination). It isnot until the ratio of silicon to nitrogen is 0.75 (i.e. thestoichiometric ratio) that the PMIC is essentially eliminated.

[0012] Thus, if one were to design the barrier to maximize resistance topositive mobile ion penetration one would use a ratio of silicon tonitrogen of 0.75. However, as shown above, this would provide very pooretch selectivity. On the other hand, if one were to design for the bestetch selectivity, one would design a nitride barrier having a ratio ofsilicon to nitrogen of 1.0 or greater; but this would provide poorresistance to positive mobile ion penetration.

[0013] According to the present invention, a barrier is provided whichwill achieve both high resistance to positive mobile ion penetration andvery good etch selectivity with respect to SiO₂. This is accomplished byproviding a barrier having two separate sections or layers. A firstlayer of silicon nitride is tailored to have excellent resistance topositive mobile ion penetration and thus has a ratio of silicon tonitrogen of less than about 0.8 and preferable about 0.75. A secondlayer of silicon nitride is provided which has a silicon to nitrogenratio of greater than about 0.8 preferably about 1.05. This will provideexcellent etch selectivity. By having a dual layer barrier as described,the barrier will provide not only good etch selectivity but resistanceto positive mobile ion contamination.

[0014] Referring now to FIGS. 4A through 4G, various steps of thepresent invention are depicted in very diagrammatic fashion. As seen inFIG. 4A a silicon substrate 10 is provided which has a gate device 12separated from the substrate 10 by means of a gate oxide layer 13. Thesubstrate has a region 14 of opposite polarity (shown as N⁺) on top ofwhich is a silicided layer 15, which silicided layer 15 also overliesthe gate 12.

[0015] A first layer of silicon nitride (Si_(x)N_(y)) 16 is depositedover the substrate 10 and the gate device 12. The first layer of siliconnitride 16 in the preferred embodiment is formed in an AME 5000 toolsold by Applied Materials, Inc. with an atmosphere of SiH₄ and NH₃ toform a silicon nitride having a ratio of silicon to nitrogen of about0.75. The ratio of silicon to nitrogen is controlled by controlling theratio of SiH₄ to NH₃ in a well known manner. Preferably this first layer16 is from about 25 to about 50 nanometers thick.

[0016] Following the deposition of the first layer 16 a second layer 18of silicon nitride is deposited over the first layer 16 as shown in FIG.4B. Again this is done in the AME 5000 tool in an atmosphere of SiH₄ andNH₃. The ratio of SiH₄ to NH₃ in forming this second layer 18 iscontrolled so as to form a silicon nitride with silicon to nitrogenratio of at least 1.0 and preferably 1.05. This layer 18 is also formedto a thickness of about 25 to about 50 nanometers so that the totalthickness of the first and second layers 16, 18 is from about 50 toabout 100 nanometers. It is not critical whether the layer 16 or 18 isformed on the substrate; however in the preferred embodiment, the layer16 is formed on the substrate 10 and the layer 18 is formed over thelayer 16.

[0017] On top of the layer 18 is deposited a layer of silicon dioxide(TEOS) 20 preferably doped with boron (BSG) or phosphorous (PSG) or both(BPSG) as shown in FIG. 4C which also is formed in a conventional manneragain using the AME 5000 tool. This layer 20 is conventionally at leastabout 0.6 microns thick.

[0018] As shown in FIG. 4D surface 22 of the TEOS 20 is coated with aphotoresist 24, which is photoimaged and developed in a conventionalmanner to provide openings one of which is shown at 26 in thephotoresist 22. One photoresist that is especially useful is positiveacting resist 5409 sold by Shipley Corp.

[0019] Following the developing of the photoresist layer 24, the SiO₂exposed through the opening 26 is anisotropically etched preferably in aCHF₃:O₂ atmosphere to form opening 28 in the SiO₂ as shown in FIG. 4E.Because of the layer 18 of Si_(x)N_(y) has a high Si to N ratio it has avery high selectivity of etch rate as compared to the silicon dioxide20, the layer 18 Si_(x)N_(y) acts as an excellent etch stop material.Never-the-less a certain amount of the layer 18 is removed as shown as29 in FIG. 4E.

[0020] Following the reactive ion etching, the remaining photoresist 24is stripped and the exposed silicon nitride layers 16 and 18 are removedby dry etching in Ar:CHF₃ to provide the structures shown in FIG. 4F.

[0021] Following the removal of the Si_(x)N_(y) layers in openings 26, acontact barrier such as TiN 30 is formed on the SiO₂ wall in opening 26and surface 22 and on the exposed substrate 10. This is followed bydeposition of a metal such as Tungsten (W) 32, as shown in FIG. 4G.

[0022] That portion of the Si_(x)N_(y) layers remaining under theSiO_(x), which have not been exposed and etched, contain the layer 16which has excelled resistance to PMIC during subsequent processing. Thusthe two layers 16 and 18 have together provided high etch selectivityduring RIE of the silicon and also reduced or eliminating PMIC duringsubsequent processing.

[0023] Of course it should be understood that the ratios of Si to N inthe two layers can be varied as can be the thicknesses of the twolayers. For example if there is more concern for either more etchselectivity or improved barrier to positive mobile ion penetration thethickness of each of the layers 16 and 18 as well as the ratios of Si toN in each layer can be varied. Also, as noted above the layer 18 withhigh etch selectivity can be formed on the substrate, and the layer 16with good resistance to PMIC can be formed on the layer 18.

[0024] Also it should be understood that in using a conventional toolfor forming the silicon nitride, it is possible to provide a barrierwhich has a gradient throughout; i.e. a structure which at the surfaceof the substrate has excellent barrier properties to positive mobile ionpenetration and then gradually increases the silicon to nitrogen ratioso that the outer surface has high etch selectivity (or vice versa).This can be accomplished by starting with a ratio of SiH₄ to NH₃ thatwill provide a ratio of 0.75 of Si to N in the silicon nitride, and thengradually changing the concentrations of SiH₄ and NH₃ such that at theend of the cycle the ratio of Si to N in the silicon nitride is 1.0 ormore.

[0025] Thus, according to the present invention an improved etch stopbarrier is provided which provides both excellent resistance to positivemobile ion penetration and also very good etched selectivity in the samebarrier by having multiple layers of material which are tailored to aspecific function.

We claim:
 1. A semi conductor structure comprising; a semi conductorsubstrate, a barrier of silicon nitride disposed on said substrate, saidbarrier of silicon nitride having a first layer and a second layer, thesilicon nitride in said second layer having a Si:N ratio greater thanthe Si:N ratio of the silicon nitride in said first layer.
 2. Theinvention as defined in claim 1 whereas the Si:N ratio in said secondlayer is at least about 0.8 and the ratio of Si:N in said first layer isless than about 0.8.
 3. The invention as defined in claim 2 wherein saidfirst layer has a ratio of Si:N of about 0.75.
 4. The invention asdefined in claim 2 wherein said second layer has a ratio of Si:N of atleast about 1.0.
 5. The invention as defined in claim 4 wherein saidfirst layer has a ratio of Si:N of about 0.75.
 6. The invention asdefined in claim 2 wherein said barrier is between 50 and 100 nanometersthick.
 7. The invention as defined in claim 6 wherein each of said firstand second layers is below about 25 and about 50 nanometers thick. 8.The invention as defined in claim 1 wherein Si:N ratio in the SiNbarrier progressively changes from said substrate through said secondlayer.
 9. The invention as defined in claim 1 wherein a layer of silicondioxide overlies said barrier of silicon nitride, and wherein at leastone opening extends through said silicon dioxide layer and said siliconnitride barrier and having a conductor disposed therein.
 10. Theinvention as defined in claim 1 wherein said first layer of siliconnitride overlies said substrate and said second layer of silicon nitrideoverlies said first layer.
 11. The invention as defined in claim 10wherein a layer of silicon dioxide overlies said second layer of siliconnitride, and wherein there is at least one opening in said silicondioxide layer and said silicon nitride, and a conductive material isdisposed in said at least one opening.
 12. A method of reactive ionetching SiO₂ comprising the steps of; providing a substrate, forming abarrier of silicon nitride on said substrate having a first layer and asecond layer, said second layer having a greater ratio of Si:N than saidfirst layer, forming a layer of SiO₂ on said barrier, and reactive ionetching at least one opening in said SiO₂ using said silicon nitridebarrier as an etch stop.
 13. The invention as defined in claim 12further comprising the steps of controlling the Si:N ratio in saidsecond layer to at least about 0.8 and the ratio of Si:N in said firstlayer to less than about 0.8.
 14. The invention as defined in claim 13wherein said first layer has a ratio of Si:N of about 0.75.
 15. Theinvention as defined in claim 13 wherein said second layer has a ratioof Si:N of at least about 1.0.
 16. The invention as defined in claim 15wherein said first layer has a ratio of Si:N of about 0.75.
 17. Theinvention as defined in claim 13 wherein said barrier is between 50 and100 nanometers thick.
 18. The invention as defined in claim 17 whereineach of said first and second layers is between about 25 and about 50nanometers thick.
 19. The invention as defined in claim 12 wherein Si:Nratio in the SiN barrier progressively increases from said substratethrough said second layer.
 20. The invention as defined in claim 12wherein said first layer of silicon nitride is deposited on saidsubstrate and said second layer of silicon nitride is deposited on saidfirst layer a silicon nitride.